The meaning
> Most birds are free to go where they want, unless they are in a cage or an aviary, in which case they are not free as a bird. Completely free to do what you want and without any worries.
I was free as a bird when I finished my final school exam
Etymology
> Unfortunately I could not find any reference on the origin of «to be free as a bird». But I can tell you it is a song originally composed and recorded in 1977 as a home demo by John Lennon. In 1995 a studio version of the recording, incorporating contributions from Paul McCartney, George Harrison and Ringo Starr, was released as a single by The Beatles, 25 years after their break-up and 15 years after the death of Lennon.
It is also an album by Supertramp or a Lynyrd Skynyrd song!
Yes I know it has nothing to do with the etymology of today’s idiomatic expression.
Wrong translation
> Être aussi libre qu’un oiseau
Correct French idiom
> Être libre comme l’air
